Of the granular materials which are utilized as abrasive particles or abrasive grain, silicon carbide is relatively inexpensive and useful but the workpieces which may be efficiently abraded by it are rather limited. Silicon carbide is commonly used to abrade cast iron, various metals and non-metallic materials. Silicon carbide is not very useful for abrading steel, largely because of its chemical reactivity with steel at the high interface temperatures generated during grinding. Various attempts to improve the steel abrading performance of silicon carbide have been made, but these have not provided significant improvement in the performance of the modified abrasive grains.
The use of various refractory materials such as silicon nitride as an abrasive material per se has its advocates and detractors, leading one skilled in the art to be unsure whether an abrasive grain made wholly or partly of silicon nitride could ever be useful for grinding steel and various other workpieces. Erasmus et al. (U.S. Pat. Nos. 2,750,268 and 2,628,896) observe that silicon nitride abrasive powders or compacts provide an excellent abrasive for abrading hard substances such as cast iron, cobalt-chromium-tungsten alloys, synthetic sapphire crystals, brass, aluminum, and others. More recently, an article entitled "An Abrasive Material Based on Silicon Nitride For Grinding Unhardened Steels" by I. I. Osipova et at., Institute of Materials Science, Academy of Sciences of the Ukrainian SSR. translated from Poroshkovaya Metalurgiya, No. 5 (1973) pp. 34-37, May, 1977 suggests that silicon nitride may be used as an abrasive material because it has a relatively high microhardness, stable physical properties and crystal structure, is easy to produce and has a fine-grain homogeneous structure. However, still more recently, Samanta (U.S. Pat. No. 4,286,905) notes that silicon nitride-based ceramic cutting tools exhibit poor resistance to wear or surface abrasion and oxidation when used to machine steel or nodular iron at elevated temperatures, e.g. 1000.degree.-1200.degree. C. This reference indicates that tools made of this material wear out readily as evidenced by severe crater wear. Samanta suggests that such poor wear resistance is an inherent defect in the silicon nitride.
Matsuno et al., in an article entitled "Possibility of Silicon Nitride as Abrasive Grain", pages 155-156, Bulletin Japan Soc. of Prec. Engg., Vol. 13, No. 3 (September 1979) acknowledge that silicon nitride may be employed as an abrasive material. Matsuno et al. report on the testing of chemical vapor deposited and pressed silicon nitride abrasive grits and conclude that the grinding capacity of silicon nitride does not exceed that of conventional abrasive grain and that it would be "unnatural to hope much from this new ceramic material as an abrasive grain."
Modified abrasive particles, some of which include a coating of silicon nitride, have been prepared. In most cases, however, such coatings served to protect the abrasive grit from attack by molten metals during formulation into abrasive articles or to provide a surface with improved adhesion. Reference which disclose coating to improve adhesion include, for example, Japanese Pat. No. 49-50008 (diamond coated with silicon nitride) and U.S. Pat. No. 3,520,667 (diamond coated with silicon carbide). Similarly, Johnson et al. (U.S. Pat. No. 4,249,913) disclose coating silicon carbide particles with alumina to prevent dissolution or diffusion of the silicon carbide in a molten metal matrix. By contrast, the present applicant has discovered that such a combination (alumina coated on silicon carbide particles) does not provide an improved abrasive material unless the silicon carbide particle has been first coated with a metal nitride and then overcoated with alumina.
Koopman (U.S. Pat. No. 3,269,815) discloses cementing finely divided particles of solid materials such as carbides and nitrides (such as silicon nitride) to silicon carbide or aluminum oxide abrasive grains by a thin film of glass. Applicant has discovered, however, that silicon carbide particles coated with glass-cemented particles of silicon nitride, as described in Koopman, provides abrasive grain which has grinding performance on steel virtually the same as uncoated, untreated silicon carbide abrasive grain, when such abrasive grain is incorporated and tested in abrasive discs.
Despite the fact that the vapor deposition of refractory materials on various substrates is well known, as exemplified by Hirai et al. (U.S. Pat. No. 4,340,568), disclosing the chemical vapor deposition of silicon nitride, and Landingham et al (U.S. Pat. No. 4,336,304), disclosing the chemical vapor deposition of sialon, applicant is unaware of any reference which teaches chemical vapor deposition of refractory coatings comprising metal nitride or carbide on silicon carbide abrasive granules. Examples of chemical vapor deposition of silicon nitride are also disclosed by D. R. Messier et al., "Silicon Nitride" in Preparation and Properties of Solid State Materials, W. R. Wilcox, ed., Volume 7, Chapter 2, Marcel Dekker Inc., New York (1982).
Examples of chemical vapor deposition of other refractory materials on cutting tools are given in Japanese Pat. Nos. 57,145,086, 57,145,087, 57,145,088, and 57,145,089, published Sept. 2, 1982.